Ultra high numerical aperture optical fibers

ABSTRACT

Various embodiments described include optical fiber designs and fabrication processes for ultra high numerical aperture optical fibers (UHNAF) having a numerical aperture (NA) of about 1. Various embodiments of UHNAF may have an NA greater than about 0.7, greater than about 0.8, greater than about 0.9, or greater than about 0.95. Embodiments of UHNAF may have a small core diameter and may have low transmission loss. Embodiments of UHNAF having a sufficiently small core diameter provide single mode operation. Some embodiments have a low V number, for example, less than 2.4 and large dispersion. Some embodiments of UHNAF have extremely large negative dispersion, for example, less than about −300 ps/nm/km in some embodiments. Systems and apparatus using UHNAF are also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of prior U.S. patent applicationSer. No. 11/691,986, filed Mar. 27, 2007, entitled “ULTRA HIGH NUMERICALAPERTURE OPTICAL FIBERS,” the entirety of which is hereby incorporatedby reference herein.

BACKGROUND

1. Field of the Invention

This application relates to apparatus and methods for optical fibersincluding, for example, optical fibers having a high numerical aperture.

2. Description of the Related Art

Significant technology developments in the late 1970s reduced opticalfiber transmission loss to below 0.2 dB per km. These developments allowtransmission of an optical signal over a few hundred kilometers withouta repeater to boost signal, which is a significant achievement overelectric cables based on metallic conductors. In addition to fibertransmission loss, another important transmission characteristic of anoptical fiber is dispersion, which describes wavelength-dependenttransmission delays. An optical pulse broadens during transmission inthe presence of dispersion. This limits transmission speed as well asdistance, because optical pulses will run into each other at the outputend of the optical fiber at high transmission speed and long distance.The overlapped optical pulses at the output of the transmission fibermay not be recovered, and the transmitted information will be lost.

Dispersion in an optical fiber has two components: material dispersionand waveguide dispersion. Material dispersion comes from thewavelength-dependent refractive index of the material used to makeoptical fibers. For example, fibers may be made from a glass such asfused silica, with possible addition of dopants such as germanium,phosphorous, fluorine, and/or boron. Since the glass is mainly silica,material dispersion varies slightly from fiber to fiber. Waveguidedispersion on another hand comes from wavelength-dependent guidingproperties of the optical waveguide and can be varied significantly by awaveguide design. Material dispersion and waveguide dispersion can havedifferent arithmetical signs, and they can substantially cancel eachother in some fiber embodiments and enhance each other in otherembodiments. Standard silica, single-mode fiber has a zero-dispersionwavelength of ˜1.3 μm, which is the wavelength at which material andwaveguide dispersion cancel each other precisely. An earlier generationof single-mode optical fiber transmission systems is based on operationat a wavelength of ˜1.3 μm, which provides low dispersion for high speedand long distance transmission.

Since the minimum transmission loss in silica fiber is at a wavelengthof ˜1.55 μm, dispersion-shifted optical fibers with a zero-dispersionwavelength of ˜1.55 μm were developed in the mid-eighties by varying thewaveguide dispersion. In the nineties, wavelength-division multiplexed(WDM) systems were developed to accommodate the rapidly growth of theInternet. In a WDM system, multiple channels located at differentwavelengths are transmitted in the same fiber. Hundreds of channels canbe used in a WDM system with components at the input and outputrespectively to multiplex and de-multiplex the large number of channels.In a WDM system, many channels may operate at wavelengths where there issignificant dispersion. In addition, it was found that the large numberof channels can increase optical intensity in the optical fiber to thepoint where different channels start to interact through nonlineareffects such as four-wave-mixing (FWM). It was also realized that acertain amount of dispersion can significantly reduce nonlinear effectssuch as FWM, because different channels can walk off each other in thepresence of dispersion. This effectively reduces interaction length.

Thus, many optical fibers are fabricated to have a certain amount ofdispersion and a large effective area to reduce nonlinear effects in WDMsystems. Systems using such fibers typically use dispersion compensationmodules (DCM) at repeaters to compensate for pulse broadening caused bydispersion. A DCM may comprise a few kilometers ofdispersion-compensating fiber (DCF), which is an optical fiber designedto have high level of waveguide dispersion with an opposite sign to thatof the transmission fiber. However, an additional issue in a WDM systemis dispersion slope, which depends on the variation in dispersion withwavelength. A disadvantage of many dispersion-compensating fibers isthat it is difficult to design a DCF with a high dispersion andappropriate dispersion slope to fully compensate dispersion for all theWDM channels. This sets a limit on the number of channels that can beused so that the channels at the two edges of the transmission windowsdo not suffer significant performance degradation due to residuedispersion.

Fiber chirped pulse amplification (FCPA) systems are often used forproducing high peak power optical pulses. In an FCPA system, initiallow-power optical pulses are stretched in time by a stretcher beforeamplification. The longer pulses have lower peak power so that thepulses may incur less nonlinear penalty in an optical fiber amplifier.After amplification, the longer pulses are then compressed to producehigher peak power optical pulses in a subsequent compressor. A stretchermay comprise an optical fiber with dispersion. More stretching isdesired for higher peak power pulses. Stretching may be quantified bythe stretching ratio, which is the ratio of the stretched pulse width ofthe pulse output from the stretcher compared to the pulse width of thepulse input into the stretcher. This stretching ratio is currentlylimited by the ability of current stretchers to match precisely thedispersion and dispersion slope of the compressor. A stretcher which canbetter match the terms of higher order dispersion with that of thecompressor can further improve current FCPA systems and produce morecompressed pulses.

In addition to dispersion, loss is another important parameter infibers, specifically in systems having a total loss budget. Designs withreduced or minimum loss penalty are strongly favored.

Thus, what is needed is optical fiber that can provide a desireddispersion and that can also provide a suitable dispersion slope at asmall optical loss. The present disclosure describes apparatus andmethods providing optical fibers that can be used in applications suchas described above as well as in other applications.

SUMMARY OF CERTAIN EMBODIMENTS

Various embodiments described herein include optical fiber designs andfabrication processes for fibers that have a higher numerical apertureand a small core size such that dispersion is pronounced. Thisdispersion may, for example, be two or three orders of magnitude largerthan material dispersion in some embodiments.

The numerical aperture can be determined by the refractive index of thecore and cladding materials. Various embodiments described hereininclude designs that provide high numerical index suitable for alsoyielding high dispersion.

In some embodiments, high dispersion can be provided at relatively smallV values. In various embodiments, high numerical aperture producesincreased waveguiding that enables operation at small V values withsufficiently strong waveguiding to yield low transmission loss. Asdescribed herein, the desired dispersion characteristics may alsoresult.

In various embodiments, high numerical aperture also enables low losssingle mode operation at very small core size in certain embodiments.Single mode operation in a step-index optical fiber may be characterizedby normalized frequency V=kρNA<2.405, where k is vacuum wave vector, ρis the core radius, and NA is the numerical aperture. The vacuum wavevector is related to the vacuum wavelength of light by k=2π/λ. For afixed value of the normalized frequency V, for example, higher NApermits use of a small core radius ρ.

This small core size is associated with a small mode size measured, forexample, by a modal field diameter (MFD) characterization system. Thesmall core size may increase optical intensity in the core andconsequently may lead to an increase of nonlinear effects, which may beadvantageous for applications requiring high levels of nonlineareffects.

Various embodiments described herein, for example, include optical fiberdesigns and related fabrication processes for ultra high numericalaperture optical fibers (UHNAF) having a numerical aperture (NA) ofabout 1. Various embodiments of UHNAF may have an NA greater than about0.7, greater than about 0.8, greater than about 0.9, or greater thanabout 0.95. Embodiments of UHNAF may have a small core diameter and mayhave low transmission loss. Embodiments of UHNAF having a sufficientlysmall core diameter provide single mode operation. Some embodiments ofUHNAF have extremely large negative dispersion, for example, less (e.g.,more negative) than about −300 ps/nm/km in one embodiment.

Various embodiments disclosed herein include fabrication methods for aUHNAF with numerical aperture close to 1. Methods for splicing a UHNAFto a conventional optical fiber are provided. In some embodiments, UHNAFmay be spliced to a conventional fiber with a modal field diameter (MFD)up to about 10 μm with reduced insertion loss.

Embodiments of UHNAF may be advantageously used in a variety of systemsand applications. For example, UHNAF having low optical loss and highdispersion may be used in a dispersion compensation module (DCM). Thehigh dispersion substantially reduces the length of fiber therebyproviding more compact DCM with lower insertions loss. Variousembodiments of UHNAF described herein enable highly nonlinear fibers(HNLF) with small MFD and low loss, which may reduce the threshold ofnonlinear devices and enable a wider range of devices based on nonlineareffects.

In various embodiments, low loss may be provided by the small core. Forexample, embodiments of UHNAF in which the core diameter is smaller thanthe wavelength of light may enable low optical loss operation. In UHNAFwith small cores, the guided optical mode may have substantial opticalpower outside the core. Less optical loss is present as less light ispropagated in the core material that introduces the loss. Opticalsensors comprising such UHNAF may also provide substantially strong andlong interaction of the probing optical energy with a material disposedoutside the core (e.g., in a cladding).

Embodiments of ultra high numerical aperture fiber, which provideincreased waveguiding, also allow fibers to operate at very small bendradius without significant bend losses. Such UHNAF embodiments can beused to wire homes and offices in order to provide high capacity networkand internet connectivity as well as for compact fiber devices.

Optical fiber amplifiers comprising ultra high numerical aperture fiberand resultant reduced mode field diameter can provide enhanced pumpintensity, leading to significantly higher gain per unit of pump powerin the optical fiber amplifier. Use of UHNAF may advantageously reducethe cost of fiber amplifiers. Many other applications andimplementations of UHNAF are possible.

A wide variety of embodiments are described herein. One embodiment ofthe invention, for example, comprises an optical fiber capable ofpropagating light having a wavelength. The fiber comprises a core, anair cladding surrounding the core, the air cladding comprising an airgap having a width, an outer layer surrounding the air cladding; and aplurality of webs mechanically coupling the core and the outer layersuch that the air gap is disposed therebetween. The fiber has (i) anumerical aperture greater than about 0.8, and (ii) a loss less thanabout 10 dB/km.

Another embodiment of the invention also comprises an optical fibercapable of propagating light having a wavelength. The fiber comprises acore having a diameter, a first cladding surrounding the core, the firstcladding comprising a gap having a width, an outer layer surrounding thefirst cladding, and a plurality of webs mechanically coupling the coreand the outer layer. The core diameter is less than about 3 micrometers,and the fiber has a loss less than about 10 dB/km.

Another embodiment of the invention comprises a method that includesproviding a preform assembly comprising an outer tube having a passageextending at least partially therethrough and one or more hollow innertubes at least partly within the passage. The hollow inner tubes havinghollow regions therein. The method further comprises expanding thehollow inner tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the disclosed inventions will now bedescribed with reference to the following drawings, which are intendedfor illustrative purposes only.

FIG. 1A is a cross-section view schematically illustrating aconventional optical fiber comprising a core with diameter 2ρ and acladding with diameter D.

FIG. 1B is a graph showing results of a simulation of dispersion versusnormalized frequency V for various step index fibers with differentnumerical aperture.

FIG. 2 is a cross-section view schematically illustrating an embodimentof an ultra high numerical aperture optical fiber (UHNAF) comprising acore, webs, first (air) cladding (having width d), and second cladding.

FIG. 3 is a graph showing results of simulations of dispersion (leftaxis) and leakage loss (right axis) versus normalized wavelength (bottomaxis) and normalized frequency (top axis) for ultra high numericalaperture fibers having different air cladding widths, d.

FIG. 4 is a graph showing simulated leakage loss versus normalized first(air) cladding thickness, d, of ultra high numerical aperture fibers.

FIG. 5A is a graph showing simulated modal field distribution versusdistance from the center of the core at various wavelengths for anexample UHNAF.

FIG. 5B is a graph showing simulated modal field diameter (MFD) versusnormalized frequency V for UHNAFs having different numerical apertures(NA).

FIG. 6A is a cross-section view schematically illustrating anotherembodiment of a UHNAF comprising a core, webs, first and secondcladdings, and first and second additional claddings.

FIG. 6B is a perspective view that schematically illustrates a UHNAF anda conventional optical fiber before splicing, where the UHNAF comprisesa transition which may be used for lowering splice loss to theconventional fiber.

FIG. 6C is a perspective view schematically illustrating a UHNAF thatcomprises a mode expansion taper.

FIG. 7 schematically illustrates a portion of a preform fabricationapparatus used in fabricating a preform for a UHNAF.

FIGS. 8A-8C are cross-section views schematically illustratingembodiments of preform assemblies used in a preform fabrication process.

FIG. 9 is a cross-section view of a post-expansion assembly resultingfrom heating and expansion of the preform assembly shown in FIG. 8A inthe preform fabrication apparatus of FIG. 7.

FIG. 10 schematically illustrates an optional heating and expansion stepthat may be applied to a post-expansion preform assembly to furtherincrease the width of the first (air) cladding.

FIG. 11 is a cross-section view that schematically illustrates twoadditional layers formed substantially uniformly on the inside surfaceof the tube used in the preform apparatus of FIG. 10. These layers maybe used to provide a UHNAF with additional claddings such as used in thetaper shown in FIG. 6B.

FIG. 12A is a cross-section view that schematically illustrates a secondexpanded assembly, which comprises a core, a first cladding, webs, and asecond cladding wherein the first (air) cladding has been furtherincreased in size using apparatus such as shown in FIG. 10.

FIG. 12B is a cross-section view that schematically illustrates a thirdexpanded assembly, which comprises a core, a first cladding, webs, and asecond cladding inserted in a tube which may be subsequently drawn.

FIG. 13A is a cross-section view that schematically illustrates a fiberdrawn from the third expanded assembly shown in FIG. 12B.

FIG. 13B is a cross-section view schematically illustrating anembodiment of a preform assembly that may be used in a preformfabrication process in which two heating and expansion steps arecombined into a single processing step.

FIG. 14A schematically illustrates a portion of a preform fabricationapparatus used in fabricating a preform for a UHNAF using vacuum.

FIG. 14B schematically illustrates a cross-section of a perform assemblyused for the caning stage of the vacuum-assisted expansion process shownin FIG. 14A.

FIG. 14C schematically illustrates a cross-section of the cane drawnduring the caning stage of the vacuum-assisted expansion process.

FIG. 14D schematically illustrates a portion of a fiber drawingapparatus used in fabricating a UHNAF using vacuum.

FIG. 14E schematically illustrates a cross-section of an assembly usedin the drawing apparatus depicted in FIG. 4D.

FIG. 14F schematically illustrates a cross-section of a fiber drawn fromthe assembly depicted in FIG. 14E.

FIG. 15 schematically illustrates a multi-span wavelength-divisionmultiplexed (WDM) system, including dispersion compensation modules(DCM) comprising UHNAF.

FIG. 16 schematically illustrates a fiber chirped pulse amplification(FCPA) system, which includes a stretcher comprising UHNAF.

FIG. 17 schematically illustrates a spool of UHNAF.

FIGS. 18A and 18B schematically illustrate embodiments of optical fibersensor systems comprising UHNAF.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A cross-section of one type of conventional optical fiber 100 isschematically illustrated in FIG. 1A. The fiber 100 comprises a core 101and a cladding 102. The fiber 100 may be further covered and/or coatedwith protective coatings, which are not illustrated in FIG. 1A. Opticalcharacteristics of conventional optical fibers are mostly determined bytwo parameters, numerical aperture (NA), and core diameter 2ρ. Numericalaperture may be defined as NA=(n_(co) ²−n_(cl) ²)^(1/2), when n_(co)²−n_(cl) ²<1, and NA=1, when n_(co) ²−n_(cl) ²≧1, where n_(co) andn_(cl) are the refractive index of the core 101 and the cladding 102,respectively. Two further dependent parameters are also useful indescribing optical characteristics of the optical fiber 100: relativerefractive index Δ=(n_(co)−n_(cl))/n_(cl) and normalized frequencyV=2πρNA/λ), where λ is the vacuum wavelength of light. FIG. 1A depictsthe diameter D of the optical fiber 100.

For a step index fiber, the fiber 100 supports only one mode when V isless than 2.405, which is referred to as the single mode operationregime. For a fixed value of the normalized frequency V, a higher NApermits use of a small core radius ρ. Many fibers are operated in thesingle mode regime in which the normalized frequency is in a range fromabout 1.4 to about 2.4. Transmission loss of the conventional opticalfiber 100 is dominated by Rayleigh scattering at shorter wavelengths andby a phonon absorption band located in the infrared.

FIG. 1B is a graph illustrating results of a simulation of dispersionversus normalized frequency in conventional step index optical fibershaving different values of numerical aperture. Curves 151-155 showdispersion results for fibers with numerical apertures of 1.0, 0.75,0.5, 0.25, and 0.125, respectively. It can be clearly seen from FIG. 1Bthat in the single mode regime where V<2.405, fibers with highernumerical aperture have larger values of negative dispersion (e.g., morenegative) as well as higher dispersion slope (e.g., variation ofdispersion with wavelength).

Advantageously, providing a higher NA can enable a smaller core radiusand small modal field diameter (MFD) while preserving single modeoperation. In the regime where the fiber 100 has a high numericalaperture and a relatively small V value (V<2.3), very high dispersioncan be achieved. Additionally, higher numerical aperture may beadvantageous for highly nonlinear fibers, which also have a small modalfield diameter. In addition, it is easy to fabricate highlypolarization-maintaining optical fibers and polarizing optical fiberswith high numerical aperture.

FIG. 2 is a cross-section that schematically illustrates an embodimentof an optical fiber 200 comprising a core 201, a first (e.g., air)cladding 202, six webs 203, and an outer layer comprising a secondcladding 204. The optical fiber 200 may be described as having anultra-high numerical aperture, because certain embodiments of the fiber200 can have numerical apertures substantially larger than the numericalaperture of a conventional fiber, such as the fiber 100 shown in FIG.1A. For example, embodiments of ultra high numerical aperture fiber(UHNAF) may have a numerical aperture of about 1. In various embodimentsof the UHNAF 200, the numerical aperture may be greater than about 0.7,greater than about 0.8, greater than about 0.9, or greater than about0.95.

In some embodiments, because the numerical aperture is large, a smallcore can be provided in a single mode fiber. This small core may resultin higher nonlinearity in certain embodiments. The higher numericalaperture also enables highly dispersive optical fibers in someembodiments. Other advantages may also be obtained. As described below,for example, loss may be reduced with the small core as much of theoptical mode extends into a gap (e.g., air gap) between the core 201 andthe second cladding 204 and because the light propagates through air,the light does not incur the level of optical loss encountered whenlight propagates through glass, plastic or other core or claddingmaterials. Other advantages may be obtained in other embodiments.

In certain embodiments, the core 201 may comprise a glass, e.g., fusedsilica or fused silica doped with one or a combination of germanium,phosphorous, fluorine, boron, aluminum, titanium, tin, and rare earthelements. In other embodiments, the glass may comprise an oxide glass, afluoride glass, and/or a chalcogenide glass. The glass may be doped withone or more dopants as described above for fused silica. In oneembodiment, the glass comprises tellurite. The first cladding 202 maycomprise vacuum, a liquid, and/or a gas. In certain embodiments, the gasis air. In some embodiments, the gas comprises inert gas. The gas mayinclude, for example, one or a combination of nitrogen, helium, andinert gases.

The webs 203 extend between the core 201 and the second cladding 204. Inthe embodiment shown in FIG. 2, each of the webs 203 extends along asubstantially radial direction. In other embodiments the webs 203 may becurved or have other shapes or configurations. The webs 203 may bearranged substantially uniformly in azimuth around the core 201, forexample, as shown in FIG. 2. Nonuniform azimuthal arrangements may alsobe used. Although six webs 203 are shown in FIG. 2, a larger or smallernumber of webs 203 can be used in other embodiments. For example, two,three, four, five, seven, eight, nine, or ten webs 203 may be used invarious embodiments. In an embodiment, one web is used.

The second cladding 204 may comprise glass, e.g., fused silica. In someembodiments, the glass may comprise an oxide glass, a fluoride glass,and/or a chalcogenide glass. The glass may be doped with one or moredopants as described above for fused silica glass. In one embodiment,the glass comprises tellurite. In other embodiments of the fiber 200,materials other than glass (or doped glass) may be used for the core201, one or more of the webs 203, the first cladding 202, and/or thesecond cladding 204. The fiber 200 optionally may be coated or covered(not shown) to reduce damage to and losses from the fiber 200.

In certain embodiments, the second cladding 204 may have a refractiveindex greater than or equal to the refractive index of the core 201. Incertain such embodiments, if the air gap (e.g., the first cladding 202)is sufficiently wide such that the optical mode does not reach thesecond cladding 204, then the second cladding 204 mainly providesstructural support and not optical functionality. In such embodiments,the “second cladding” 204 technically may not be a cladding,

FIG. 2 depicts a parameter d, which is defined as the width of the firstcladding 202. The width, d, of the first cladding layer is shown in FIG.2 as the width of the gap or space extending from the outer edge of thecore 201 to the inner surface of the second cladding 204. The webs 203shown in FIG. 2 have a radial length that is approximately equal to dand a transverse (e.g., azimuthal) thickness that is much smaller thand. The thickness of the webs 203 advantageously may be made sufficientlysmall in order to achieve a sufficiently high value of NA andsufficiently low transmission loss. In some embodiments, the webs 203have a transverse thickness that is much smaller than a typicalwavelength of light propagated in the fiber 200, for example, less thanabout one tenth of the wavelength. In some embodiments, the thickness ofthe web 203 is less than about 150 nm. In other embodiments, thethickness can be smaller such as, for example, less than about 100 nm.

Optical performance of the optical fiber 200 can be simulated withoutconsidering the impact of the webs 203 on the propagation of light inthe fiber 200. In these simulations, the transverse thicknesses of thewebs 203 are assumed to be sufficiently thin compared to the wavelengthof light so that the webs 203 do not have a significant effect on theoptical performance of the fiber 200. If the diameter of the core 201 isvery small, a substantial amount of optical energy propagates in thefirst cladding 202 and extends some distance away from the core 201.Leakage loss due to the interaction of the optical energy and the secondcladding 204 is considered in the simulation for various widths of thefirst cladding 202. The simulation is performed with a mode solver basedon a multipole algorithm such as, for example, the algorithm describedin “Multipole method for microstructured optical fibers. I. Formulation”by White, et al., Journal of Optical Society of America B, vol. 19, pp.2322-2330 (2002) and “Multipole method for microstructured opticalfibers. II. Implementation and results” by Kuhlmey, et al., Journal ofOptical Society of America B, vol. 19, pp. 2331-2340 (2002), the entiredisclosure of each of which is hereby incorporated by reference herein.The Maxwell electromagnetic equations governing light propagation in anoptical waveguide can be rescaled as long as all dimensional parametersare scaled in the same way. Accordingly, all dimensional parameters arescaled relative to the core diameter 2ρ.

In the following example simulations, the core 201 and the secondcladding 204 are assumed to be made of fused silica with a refractiveindex of 1.45. The first cladding 202 is assumed to be air with arefractive index of 1. These values of the refractive indices yield anNA of 1 for the fiber 200. Optical performance of the fiber 200 issimulated for values of the normalized first cladding width, d/2ρ, equalto 5, 7.5, 10, 12.5, and 15. FIG. 3 is a graph that shows simulateddispersion and leakage loss of the fiber 200 versus normalizedwavelength λ/2ρ. The normalized frequency V is indicated at the top ofthe graph in FIG. 3. Dispersion (measured in units of ps/nm/km) isindicated on the left vertical axis of the graph, and leakage loss(measured in units of dB/km) is indicated on the right vertical axis.

Dispersion changes very little for different values of the firstcladding width d/2ρ. As can be seen from FIG. 3, individual dispersioncurves for each value of d/2ρ overlap into a single curve 300.Dispersion, however, changes noticeably between both positive andnegative dispersion with change in core diameter, 2ρ. FIG. 3 shows thatthere are several regimes in this dispersion curve 300. For relativelylarge core diameters, λ/2ρ<0.7, the fiber 200 exhibits large positivedispersion with a positive dispersion slope. Around λ/2ρ≈0.7, the fiber200 exhibits a maximum positive dispersion of about 425 ps/nm/km withvery small dispersion slope. For core diameters in a range 0.7<λ/2ρ<1.3,the fiber 200 exhibits a positive dispersion with a negative dispersionslope. Around λ/2ρ≈1.3, the fiber 200 exhibits very small dispersionwith a negative dispersion slope. In a range of core diameters where1.3<λ/2ρ<2.2, the fiber 200 exhibits a negative dispersion with anegative dispersion slope. Around λ/2ρ≈2.2, the fiber 200 exhibits amaximum negative dispersion of about −1371 ps/nm/km and a very lowdispersion slope. For relatively small core diameters, λ/2ρ>2.2, thefiber 200 exhibits negative dispersion with a positive dispersion slope.

The normalized frequency value of this embodiment of the fiber 200 isV=π/(λ/2ρ), because NA=1. As discussed above, V is less than 2.405 forsingle mode operation of the fiber 200, which corresponds to λ/2ρ>1.306.Therefore, embodiments of the UHNAF 200 provide single mode operation ifthe core 201 has a sufficiently small diameter: 2ρ<0.7655λ. UHNAF 200with larger diameter cores can provide multimode operation. In thesingle mode regime, FIG. 3 shows that this embodiment of the ultra highnumerical aperture fiber 200 will have negative dispersion. The highestnegative dispersion slope of the dispersion curve 300 occurs nearλ/2ρ≈1.6, which is in the single mode operating regime.

FIG. 3 shows that leakage loss of the fiber 200 varies significantlywith normalized first cladding width d/2ρ. Curves 301, 302, 303, 304,and 305 are simulated leakage losses for fibers 200 having normalizedfirst cladding widths equal to 5, 7.5, 10, 12.5, and 15, respectively.For relatively smaller values of the normalized first cladding width,the leakage loss of the fiber 200 is considerable higher than forrelatively larger values of d/2ρ (e.g., the curve 301 lies above thecurve 302, which lies above the curve 303, etc.). If a maximum leakageloss of 0.1 dB/km is acceptable in certain applications, embodiments ofthe fiber 200 can be operated at normalized wavelengths λ/2ρ up to about1.57, 1.87, 2.07, 2.25, and 2.36 for d/2ρ=5, 7.5, 10, 12.5 and 15,respectively. As described above, a possible explanation for the lowleakage loss for large gap or first cladding widths, d, is to reducemodal power tunneling into second cladding 204, which can besubsequently lost to coating materials. For smaller gap or firstcladding widths, d, the optical mode interacts more strongly with thesecond cladding 204, which may lead to increased leakage loss.

FIG. 4 also illustrates the dependence of the leakage loss versusnormalized first cladding width d/2ρ. Curves 401, 402 and 403 show theleakage loss if the fiber 200 is operated at λ/2ρ=1.748, 2.015, and 2.5,respectively. Each of the curves 401-403 is for single-mode operation ofthe fiber 200. FIG. 4 shows that leakage loss at a given normalizedwavelength λ/2ρ decreases nearly exponentially with an increase of d/2ρ.This strong dependency enables various fibers designed as describedherein to advantageously provide substantially reduced optical loss.

FIG. 5A shows results of simulations of the normalized electric fielddistributions of the above-described embodiment of the fiber 200. Curves501, 502, 503, 504, and 505 show normalized field distribution as afunction of radial distance from the center of the fiber 200 forλ/2ρ=0.5, 1.0, 1.5, 2.0, and 2.5, respectively. The curves 501-505 arecalculated assuming the core diameter 2ρ=1 μm. FIG. 5A shows that theoptical field extends substantially away from the core 201, especiallyat larger λ/2ρ. As described above, this extended field is advantageousfor reducing optical loss as much of the optical mode propagates in air,vacuum, or gas, instead of in glass, plastic, sapphire, or other morelossy material. This extended field is also advantageous for materialsensor applications, where strong interaction of the optical field andthe material to be sensed is desirable. In certain embodiments, forexample, the optical properties of fluid, such as gases or liquids,within the gap may be probed using the light propagating within theoptical fiber. Sensors comprising UHNAF are described below in furtherdetail.

FIG. 5B shows simulations of modal field diameter (MFD; measured inmicrometers) versus normalized frequency V for the fiber 200 operated ata wavelength of 1 μm. Curves 510, 511, 512, and 513 are simulations forfibers 200 having NA=1, 0.75, 0.5, and 0.25, respectively. FIG. 5B showsthat a minimum MFD is reached around V≈2, which is in the single modeoperating regime. FIG. 5B also shows that the minimum MFD is smaller foroptical fibers having higher numerical aperture.

In various embodiments of UHNAF, core diameters can be in a range fromabout λ/4 to about 5λ. The width d of the first cladding 202 can rangefrom about p to about 50ρ in some embodiments. In certain embodiments ofthe fiber 200 having smaller core diameter, the width of the firstcladding 202 is selected to be sufficiently large to provide low lossand/or large negative dispersion. In certain preferred embodiments ofUHNAF, the width of the second cladding 204 is selected to providesufficient mechanical support for the fiber 200. In other embodiments,other dimensions of the fiber 200 can be used.

FIG. 6A is a cross-section schematically illustrating another embodimentof an ultra high numerical aperture optical fiber 600. The fiber 600 mayadvantageously be used to reduce splice loss when spliced to aconventional optical fiber having a larger core diameter. The fiber 600may be generally similar to the fiber 200 and comprises a core 601, afirst cladding 602, webs 603, and a second cladding 604. The opticalfiber 600 may have a numerical aperture (NA) of about 1. In variousembodiments, the fiber 600 may be configured to have NA>0.7, NA>0.8,NA>0.9, or NA>0.95.

The fiber 600 shown in FIG. 6A also comprises additional claddings 605and 606. The first additional cladding 605 may comprise a material witha refractive index approximately equal to or slightly less than therefractive index of the core 601. The second additional cladding 606 maycomprise a material with a refractive index smaller than that of thefirst additional cladding 605. In one example of the fiber 600, the core601 comprises fused silica or fused silica doped with germanium,phosphorous or a mixture of germanium and phosphorous to give it ahigher refractive index than that of fused silica. The first claddinglayer 602 may comprise vacuum or a gas such as air, nitrogen, argon,and/or other suitable inert gases. The first additional cladding 605 maycomprise fused silica or fused silica doped with germanium, phosphorousor a mixture of germanium. The second additional cladding 606 maycomprise fused silica doped with fluorine or boron to give it a lowerrefractive index than fused silica. In other embodiments of the fiber600, the following dopants or a combination of the following dopants canbe used in the core 601 or any part of the claddings 604-606: germanium,phosphorous, boron, fluorine, tin, lead, aluminum, and rare earth ions.The webs 603 may also comprise fused silica with or without the additionof suitable dopants. Although six webs 603 are depicted in FIG. 6A,fewer or greater numbers of webs 603 may be used in other embodiments ofthe fiber 600. Additionally, as discussed above with reference to FIG.2, the configuration of the webs 603 may be different than shown in FIG.6A (e.g., the webs 603 may be curved and/or asymmetrically disposedabout the core 601, etc.). In an alternative arrangement, additionalcladdings 605 and 606 can be omitted. In such cases, the core 601 may bedoped to have a higher refractive index than cladding 604.

FIG. 6B is a perspective view that schematically illustrates the fiber600 and a conventional optical fiber 650 before splicing. Theconventional optical fiber 650 comprises a core 651 and a cladding 652.The conventional fiber 650 may comprise a step-index fiber, agraded-index fiber, or any other suitable optical fiber. Theconventional fiber 650 may be generally similar to the fiber 100 shownin FIG. 1A, or it may be any other type of optical fiber. In theembodiment shown in FIG. 6B, the core 651 of the conventional fiber 650is substantially larger than the core 601 of the fiber 600.

In one embodiment of a method for splicing the fiber 600 to the fiber650, a section of the fiber 600 is heated (e.g., by an electric arc)before splicing in order to reduce or substantially eliminate the firstcladding 602. After heating, the section may have a reducedcross-sectional area (e.g., as shown in FIGS. 6B and 6C). In someembodiments of this method, the heated section of the fiber 600 fuses,melts, or collapses into a substantially solid structure withsubstantially total elimination of the air gap or first cladding 602. Incertain embodiments, the first cladding 602 is totally eliminated by theheating.

The heated section of the fiber 600 may be cleaved, for example, nearthe center of the heated and collapsed section of the fiber 600. Aportion of the heated section near the cleave 626 comprises a transition625 schematically illustrated in FIG. 6B. The diameter of the transition625 may taper from the diameter of the unheated fiber 600 to thediameter of the heated section at the cleave 626. It is advantageous ifthe diameter of the transition 625 at the cleave 626 is substantiallyequal to the diameter D of the fiber 650. This is, however, notcritical. In embodiments in which the webs 603 have a substantiallysmall cross-sectional area (e.g., due to their relatively smalltransverse thickness), the contribution of the material in the webs 603may be neglected in determining the cross-sectional area of the heatedsection.

In one embodiment of this splicing method, the core 601 of the fiber 600remains substantially intact during the heating/collapsing process.During heating, the first cladding 602 and first additional claddings605, second additional cladding 606 and the second cladding 604 aretransformed into an outer core 610, a first transition cladding 611, anda second transition cladding 612, respectively.

FIG. 6B schematically illustrates propagation of an optical mode 620 ain the fiber 600 and 620 b in the transition 625. While propagating inthe fiber 600, the mode 620 a is confined substantially to the core 601.In the transition 625, the refractive index and diameter of the core 601may be appropriately chosen so that the core 601 does not form astrongly confining waveguide for the optical mode 620 b as the opticalmode 620 b propagates in the outer core 610 of the transition 625. Forexample, the transition 625 may be configured so that V<1, in certainembodiments. Instead, the optical mode 620 b expands at the transitionas schematically depicted in FIG. 6B. Expansion of the mode will causenegligible loss if an adiabatic condition is satisfied along thetransition 625, e.g., the transition tapers more slowly than what alocal optical mode can follow. For example, in some embodiments, localtaper angle of the transition 625 is selected to be less than the localdiffraction angle, which may be very large for confined modes.Accordingly, the adiabatic condition is relatively easy to satisfy insuch embodiments, and a wide range of tapers may be used. In thetransition 625, the optical power of the mode 620 b is substantiallyguided in the core 610, which is substantially surrounded by the firsttransition cladding 611. The diameters of the core 601 and the firstadditional cladding 605 of the fiber 600 may be chosen so that the modefield diameter (MFD) in the outer core 610 of the transition 625 isapproximately equal to the MFD of the conventional optical fiber 650.The above-described method advantageously permits a low loss splice tobe performed between the transition 625 at the cleave 626 and theconventional optical fiber 650, due to the substantially close match ofthe mode field diameters.

In another embodiment schematically illustrated in FIG. 6C, a portion680 of the fiber 600 may be tapered to have a smaller diameter. A guidedmode 630 a propagating toward the tapered portion 680 expands (asschematically depicted at 630 b) into an expanded mode 630 c thatpropagates in the tapered portion 680. The expanded mode 630 c contractsinto a guided mode 630 d as it propagates out of the tapered portion680. In order to avoid significant optical losses, the diameter of thetapered portion 680 may be configured to change according to anadiabatic condition, as described above. In some applications, thetapered portion 680 is heated and collapsed substantially as describedabove, before spicing to a conventional fiber 650. Also, the fiber 200described with reference to FIG. 2 that does not include the additionalcladding layers 605, 606 can be tapered and/or collapsed in order toexpand its guided mode. The tapered section of the fiber 200 may beheated, collapsed, and cleaved before splicing to a conventional opticalfiber. In applications where the tapered portion 680 is substantiallycollapsed, the core 201, 601 may have a higher refractive index than theimmediate cladding in the collapsed section (e.g., cladding 605), whichcan be achieved by, e.g., doping the core material. For example, a corecomprising silica may be doped with one or a combination of germanium,phosphorus, tin, etc. In such cases, the collapsed diameter of the corealong length of the fiber will expand the mode diameter of the fiber, asfiber goes from high NA in the un-collapsed fiber to a much lower NAfiber of the collapsed section. This expansion of the mode diameter maybe present even when the change in core diameter is not substantial, asillustrated in FIG. 5B. If only part of the core is doped to raise therefractive index of the core, the reduction of core diameter in thecollapsed part can further expand the optical mode in the collapsedpart. This mode expansion can also be achieved with tapering. Tailoringof the core diameter at the collapsed and/or tapered section can be usedto match the optical mode diameter in fiber 650 to reduce splice loss.One more method to vary the optical mode diameter at the collapsedand/or tapered section is by varying the refractive index of the dopedpart of the core. This variation in refractive index will effectivelyvary the NA of the fiber at the collapsed and/or tapered section. Thisapproach can also be used to match optical mode diameter to fiber 650.The fiber at the collapsed and/or tapered section is then cleaved andspliced to fiber 650.

In other embodiments of the splicing method described above, the fiber600 or the fiber 200 may be spliced (or otherwise optically coupled) toany other type of fiber, including the fibers 200, 600, dual-clad ormultiple-clad fibers, polarization-maintaining fibers, single- ormulti-mode fibers, photonic crystal fibers, etc.

Fabrication methods will now be described for making preforms for theultra high numerical aperture fibers described herein. In variousembodiments, the fabrication methods may include one or more processingsteps (which may be repeated) described with reference to FIGS. 7-14. Insome embodiments of the fabrication methods, additional processingtechniques known in the art are included.

FIG. 7 schematically illustrates a portion of a preform fabricationapparatus 750. The apparatus 750 comprises a preform chuck 705 thatholds a tube assembly 775 such that a portion of the tube assembly 775is disposed in a furnace 706. The furnace 706 comprises heating elements707, which are configured to provide a precise, controlled temperatureto the portion of the tube assembly 775 within the hot zone of thefurnace 706. The preform chuck 705 is configured to move the tubeassembly 775 through the furnace 706. In an initial phase of thefabrication process shown in FIG. 7, an end 703 of the tube assembly 775is disposed within the furnace.

The tube assembly 775 comprises a tube 700 and a stack 701 of capillarytubes disposed substantially within the tube 700. The stack 701 maycomprise capillary tubes that include a substantially hollow lumen aswell as tubes that are partially or completely solid. A small portion710 of the stack 701 may extend outside the tube 700 as shown in FIG. 7.The arrangement of the stack 701 of capillary tubes within the tube 700will be described further below with reference to FIGS. 8A-8C. Apressurizing tube 704 may be coupled to the portion 710 of the stack 701in order to form a substantially air-tight connection so that pressurecan be applied to the capillaries. The pressurizing tube 704 maycomprise a polymer such as, for example, polyimide, and the tube 704 maybe adhesively coupled to the stack 701 with a high temperature epoxy.

In some embodiments, the end 703 of the tube 700 is tapered and madesubstantially air-tight. A corresponding taper (not shown in FIG. 7) ismade at an end of the stack 701 of capillary tubes, so that the taperedends of the stack 701 and the tube 700 mate after insertion of the stack701 into the tube 700. In a preferred embodiment, the taper at the endof the stack 701 provides a substantially air-tight seal so that some orall of the capillary tubes may be pressurized.

FIG. 7 illustrates an initial phase of the preform fabrication process.The tube assembly 775 is held by the preform chuck 705, with the taperedend 703 of the tube 700 in the furnace 706. An appropriate pressure isapplied through the pressurizing tube 704 to expand the capillary tubeslocated in the hot zone created by the heating elements 707. Thepressure is selected to reduce or minimize expansion of the portion ofthe tube 700 that is in the furnace 706. In certain embodiments, one ormore inert gases are used in the pressurization system, while in otherembodiments, nitrogen, oxygen, and/or other gases can be used.

The temperature of the furnace 706 is controlled to allow a preciseexpansion of the capillary tubes in the stack 701. The preform chuck 705then relatively slowly translates the tube assembly 775 through the hotzone of the furnace 706. The translation may be at a substantiallyconstant speed to obtain a preform with a substantially long length anda substantially uniform cross-section.

Tube assemblies 775 having various arrangements of the capillary stack701 can be used in the preform fabrication apparatus 750. FIGS. 8A-8Cschematically illustrate example cross-sectional arrangements of thecapillary stack 710 within the tube 700. In FIG. 8A, a tube assembly 800comprises a tube 700 including six capillary tubes 802 disposed around acentral rod 801 inside the tube 700. The assembly 800 may be used for anultra high numerical optical fiber comprising six webs (e.g., for thefibers 200 and 600). In FIG. 8B, an assembly 810 comprises a tube 700including four capillary tubes 812 disposed around a central rod 811inside the tube 700. The assembly 810 may be used for optical fiberscomprising four webs. In FIG. 8C, the assembly 820 comprises a tube 700including three capillary tubes 822 disposed around a central rod 821inside tube 700. The assembly 820 may be used for an optical fibercomprising three webs. Assemblies having a smaller or larger number oftubes 802, 812, and 821 may be used. In some preferred embodiments, thecentral rod is substantially solid, and each of the capillary tubescomprises a substantially hollow lumen. In other embodiments, some orall of the capillary tubes may be solid, and one or more central rodsmay be used. Many variations are possible.

In the embodiments shown in FIGS. 8A-8C, the capillary tubes (802, 812,or 822) and the central rod (801, 811, or 821) are arranged to besubstantially closely packed within the tube 700. A closely packedarrangement has advantages such as reducing or minimizing relativemovement among the tubes and rods, reducing breakage of the tubes androds, and keeping the central rod substantially centered within the tube700. As can be seen from FIGS. 8A-8C, as the number of capillary tubesincreases in these embodiments, the diameter of the central rodincreases relative to the inside diameter of the tube 700 and theoutside diameter of the capillaries.

In one example of the preform fabrication process, the apparatus 750shown in FIG. 7 was used to heat and expand the tube assembly 800 shownin FIG. 8A. Prior to the heating and expansion process, the capillarytubes 802 and the central rod 801 had outside diameters of about 1.2 mm.Each capillary tube 802 had an inner diameter to outer diameter ratio ofabout 0.9. The tube 700 had an inner diameter of 3.63 mm and an outerdiameter of 4.7 mm. Argon was used as the pressurizing gas in theapparatus 750, and the pressure was 2.05 pounds/square-inch (psi) higherwithin the capillary tubes 802. The pressure within the tube 700remained at approximately ambient pressure. Accordingly, thedifferential pressure is 2.05 pounds/square-inch (psi). The pressure inthe capillary tubes 802 is increased relative to the pressure in thetube 700 so that the capillary tubes 802 expand in size within the tube700. The temperature of the furnace 706 was 1700° C. The preform chuck705 moved the tube assembly 800 at a substantially uniform speed ofabout 4 mm/min through the furnace 706 to produce a first post-expansionpreform assembly 900.

FIG. 9 is a cross-section view of the first post-expansion assembly 900resulting from heating and expansion of the assembly 800. The firstpost-expansion assembly 900 comprises a core 901 that is surrounded by afirst cladding 902. The first cladding 902 has six webs 903. The tube700 has slightly expanded to form a second cladding 904.

In some embodiments of the fabrication method, the first post-expansionassembly 900 can be drawn into an ultra high numerical aperture fiber.In some of these methods, the first cladding 902 is pressurized whilethe fiber is drawn. In other embodiments, the first post-expansionassembly 900 is repeatedly processed an additional one, two, three, ormore times in the preform apparatus 750 to produce a second (or third,fourth, etc.) post-expansion assembly. Some of these subsequentprocessing steps will be described below. When a post-expansion assemblyhas achieved characteristics desired for a particular application, thepost-expansion assembly may be drawn into UHNAF.

FIG. 10 schematically illustrates a subsequent, optional processingphase that may be applied to the first post-expansion preform assembly900. In this phase, the expanded assembly 900 is disposed in a tube 1001with a larger diameter. The tube 1001 is held by the preform chuck 705.A taper may be made to the bottom end of the assembly 900 to form asubstantially air-tight seal before insertion into the tube 1001, whichhas a corresponding taper 1002 (see FIG. 10). The taper at the end ofthe expanded assembly 900 mates with and rests inside the taper 1002,advantageously allowing the assembly 900 to be partially supported bytube 1001.

The first expanded assembly 900 is connected to the pressurizing tube704, where pressure can again be applied. While pressure is applied andthe furnace 706 operates at an appropriate, controlled temperature, theassembly 900 is relatively slowly fed through the hot zone created bythe heating element 707. During this phase, the first expanded assembly900 further expands in the tube 1001 to form a second expanded assembly1200.

FIG. 12A is a cross-section view that schematically illustrates thesecond expanded assembly 1200, which comprises a core 1201, a firstcladding 1202, webs 1203, and a second cladding 1204. The secondcladding 1204 has expanded further away from the core 1201 and thetransverse dimension of the webs 1203 has been further reduced. Thesecond heating and expansion phase can be repeated additional times asneeded to further expand the dimension of the first cladding 1202. Thepreform assembly additionally and optionally can have its outer diameterreduced by drawing on a cane tower before repeating the second expansionstep.

In an example of the second heating and expansion phase, the firstexpanded assembly 900 was inserted into the tube 1001, which had aninner diameter of 22.5 mm and outer diameter of 25 mm. The temperatureof the furnace 706 was 1680° C. The preform chuck 705 translated theassembly 900 at a substantially uniform rate of 8 mm/min. The assembly900 was pressurized to 19 psi.

Further, optional processing steps may be used to provide additionalcladding layers. For example, to make a preform for the optical fiber600 shown in FIG. 6A, one or more additional layers of materials can beformed on an inside surface of the tube 1001 before the first expandedassembly 900 is disposed therein. FIG. 11 is a cross-section view thatschematically illustrates a relatively low refractive index layer 1101and a slightly higher refractive index layer 1102 formed substantiallyuniformly on the inside surface of the tube 1001. In one method, theseadditional layers can be formed by placing the tube 1001 in a modifiedchemical vapor deposition system such that the layers 1101 and 1102 aredeposited on the inner surface of the tube 1001. In some embodiments,the layer 1101 may be silica doped with boron or a combination offluorine and phosphorus. The layer 1102 may be fused silica or fusedsilica doped with one of, or a mixture of, germanium, phosphorous,boron, and fluorine to achieve a refractive index slightly higher thanthat of the layer 1101. In some processes, the thickness of the layer1102 (and/or the layer 1101) is controlled so that the modal fielddiameter of an optical mode in the core 610 of the transition 625 of thefiber 600 shown in FIG. 6B can be substantially matched to that of theconventional fiber 650, as described above. Although FIG. 11 shows twoadditional layers 1101 and 1102, in other embodiments of the fabricationmethod, one, three, four, five, or more layers may be formed. In someembodiments, no additional cladding layers are utilized, and thisprocessing step may be omitted.

In a third phase of the preform fabrication process, the diameter of thesecond expanded assembly 1200 may be reduced on a caning tower. Theassembly 1200 is then inserted into a tube 1211 to form a third assembly1210 as illustrated in FIG. 12B. In some embodiments of the fabricationmethod, the third assembly 1210 is drawn into optical fibers. In some ofthese embodiments, the first cladding 1202 is pressurized while thefiber is drawn. FIG. 13A is a cross-section view that schematicallyillustrates a fiber 1300 resulting from this process. The fiber 1300comprises a core 1301, a first cladding 1302, webs 1303, and a secondcladding 1304. The fiber 1300 can be configured to have an outerdiameter approximately equal to that of standard telecommunicationfibers in applications where the fiber 1300 is intended to be used withtelecommunication fibers. The fiber 1300 can be fabricated to have otherdiameters. If further expansion of the preform is desired, the secondexpanded assembly 1200 can be inserted into a tube 1211 that has alarger inner diameter than the outer diameter of the second expandedassembly 1200.

In alternative embodiments of the preform fabrication method, some orall of the first and/or the second expansion phases (and repeatsthereof) may be performed as a single process. In such embodiments, apreform assembly 1400 having a cross-section shown in FIG. 13B may beused in the expansion phase schematically illustrated in FIG. 10. Incertain such embodiments, the capillaries 802 are pressurized while theassembly 1400 moves relatively slowly through the hot zone of thefurnace 706. A skilled artisan will recognize that preform assemblieshaving cross-sections different than shown in FIG. 13B can be used aswell.

In an alternative embodiment of a method for fabricating UHNAF preformsand fibers, vacuum and optionally pressure are used in one or moreexpansion phases. In one embodiment of the vacuum-assisted fabricationprocess, a preform assembly is drawn into a cane (see FIGS. 14A-14C) andthen the cane is drawn into a fiber (see FIGS. 14D-14F).

FIG. 14A schematically illustrates a portion of the preform fabricationapparatus 750 suitable for use with embodiments of the vacuum-assistedmethod. The stack 701 is inserted into the tube 700. In someembodiments, the stack 701 is longer than the tube 700, so a portion ofthe stack 701 is extends outside the tube 700 as shown in FIG. 14A. Ataper is made at the other end, to substantially seal the stack 701 andtube 700. A taper 1002 is made at one end of a larger tube 1001. Thestack 701 and the tube 700 are disposed in the tube 1001 to formassembly 2075. The tube 1001 may be longer than the tube 700 but shorterthan the stack 701. A seal 2030 is placed at the open end of tube 1001.For example, the seal 2030 may be glued to tube 1001 by a hightemperature epoxy to form a substantially air-tight seal. The seal 2030may have a substantially central hole to allow the stack 701 to extendtherethrough. The seal 2030 may also have an offset hole 2032 to connectto a vacuum tube 2031, which is used to depressurize the tube 700 and/orthe tube 1001 as further described below. In some embodiments of theapparatus 750, a substantially air-tight seal is provided by gluing oneor both of the vacuum tube 2031 and the stack 701 with high temperatureepoxy. Optionally, the pressure tube 704 may be glued to the stack 701and used to provide pressure to the capillary tubes 802 if desired(e.g., substantially as described with reference to the embodimentsshown in FIGS. 7 and 10). The pressure tube 704 is not used in certainembodiments.

The seal 2030 may be made of a high temperature polymer such as, forexample, polytetrafluorethylene (PTFE). In some embodiments, one or bothof the vacuum tube 2031 and the optional pressure tube 704 may beflexible. The vacuum tube 2031 may be connected to a vacuum pump (suchas a variable vacuum pump), and the optional pressure tube 704 may beconnected to a pressure controller.

In one embodiment of the fabrication method, the tubes 700 and 1001 aredepressurized through the vacuum tube 2031 to create at least a partialvacuum in the tubes 700 and 1001. The capillary tubes 802 in the stack701 expand due to the pressure differential between the capillaries 802and the tube 700. Optionally, the capillaries can be further pressurizedthrough the pressure tube 704 to provide an increased pressuredifferential. In some embodiments, expansion of the capillary tubes 802causes expansion of the tube 700 within the larger tube 1001. In certainembodiments, the larger tube 1001 does not expand significantly duringthe fabrication process.

As schematically shown in FIG. 14A, the assembly 2075 is held by thechuck 705 on the preform apparatus 750, which includes the furnace 706with a hot zone provided by the heating elements 707. The assembly 2075is translated relative to the hot zone such that a cane 2001 is drawnfrom the apparatus 750. One possible advantage of using the vacuummethod described with reference to FIG. 14A is that the fabricationprocess is relatively easy to control. Another possible advantage isthat expansion of the tube 1001 can be reduced or minimized during theprocess.

FIG. 14B schematically illustrates an example cross-section of theassembly 2075. In this embodiment, the stack 701 comprises agermanium-doped rod 1901 with a graded refractive index, a peaknumerical aperture of 0.25 if included in a fused silica cladding, and adiameter of 1.19 mm. The rod 1901 is surrounded by six capillary tubes1902, each having a diameter of 1.19 mm and aninner-diameter-to-outer-diameter ratio of 0.9. Although six capillarytubes 1902 are used in the embodiment shown in FIG. 14B, fewer orgreater number of tubes 1902 are used in various other embodiments.Also, in some embodiments, the tubes 1902 are not substantially similarto each other and may have different diameters and/orinner-diameter-to-outer-diameter ratios or other properties. In oneembodiment, the tube 700 has an inner diameter of 3.6 mm and an outerdiameter of 7.2 mm. The tube 700 is disposed within the larger tube1904, which has an inner diameter of 12.5 mm and an outer diameter of 25mm, although other diameters are used in other embodiments. In someembodiments, the inner surface of the tube 1904 optionally may includeone or more layers coated or deposited thereon, which may be used toprovide additional claddings for the UHNAF. For example, in oneembodiment, the tube 1904 comprises a fluorine and phosphorus dopedlayer 1903, which has a thickness of about 0.7 mm. The layer 1903 may bedeposited using a modified chemical vapor deposition system or by someother suitable technique. In other embodiments, the layer 1903 is notdoped or is doped with different substances. The layer 1903 may have adifferent thickness in other embodiments or not be used at all.

FIG. 14C schematically illustrates an example of the cross-section ofthe cane 2001, which can be drawn from the apparatus 750 using thepreform shown in FIG. 14B. In this embodiment, the cane 2001 comprises acore 2101, first cladding 2102, webs 2103, first additional cladding2104, second additional cladding 2105, and outer cladding 2106. Any ofthese layers may have a higher or lower refractive index than that ofthe core in certain embodiments. Accordingly, layers referred to hereinas cladding layers may instead be substituted with layers having ahigher refractive index, for example, than the core and thus not operateas a optical cladding.

After the cane 2001 is fabricated, the cane 2001 may be furtherprocessed into a UHNAF. FIG. 14D schematically illustrates an embodimentof an apparatus 750 for drawing a fiber 2201 using vacuum-assistedtechniques. An assembly 2150 comprises the cane 2001 and a tube 2101.The cane 2001 may be substantially similar to the cane 2001 shown inFIG. 14C. In some embodiments, the cane 2001 is fabricated usingapparatus and methods similar to those described with reference to FIGS.14A-14C. The cane 2001 is inserted into a tube 2101. The tube 2101 maybe shorter than the cane 2001 so that a portion of the cane 2001 extendsoutside of the tube 2101. A seal 2130 is provided on one end of the tube2101, for example, by gluing a high temperature epoxy to the end of thetube 2101 to form a substantially air-tight seal. The seal 2130 mayinclude a substantially central hole that permits an end of the cane2001 to extend beyond the tube 2101 and to connect to an optionalpressure controller for pressurizing the cane 2001. The seal 2130 mayinclude an offset hole 2132, which allows connection to a vacuum tube2131, for reducing the pressure within the tube 2101. In someembodiments, one or both of the cane 2001 and the tube 2131 are glued bya high temperature epoxy to form a substantially air-tight seal. Anoptional pressure connection tube 704 may be attached to cane 2001(e.g., via high-temperature epoxy) and used to optionally pressurize thecane 2001. The pressure tube 704 is not used in some embodiments.

The assembly 2150 is held by the chuck 705 on the fabrication apparatus750, which includes the furnace 706 having a hot zone generated with theheating elements 707. In a preferred embodiment, when vacuum is appliedvia the vacuum tube 2131 to reduce the pressure within the tube 2101,the cane 2001 expands to substantially fill the space between the tube2101 and the cane 2001. The assembly 2150 is translated relative to thehot zone within the furnace 706, and a fiber 2201 is drawn from theportion of the assembly 2150 heated within the hot zone. In someembodiments, vacuum is applied to the tube 2101 and the cane 2001 is notpressurized, whereas in other embodiments, vacuum is applied to the tube2101 and the cane is pressurized. Accordingly, by suitably adjusting therelative pressures in the tube 2101 and/or the cane 2001, the fiberdrawing process advantageously can be precisely controlled.

FIG. 14E schematically shows an example cross-section of the assembly2150 that may be used in the apparatus 750 of FIG. 14D. In thisembodiment, the cane 2001 has an outer diameter of 2.8 mm. Similarly asshown in FIG. 14C, the cane 2001 may comprise a core, first cladding,webs, additional (optional) claddings, and an outer cladding. In theembodiment shown in FIG. 14E, the tube 2101 has an outer diameter of16.7 mm and an inner diameter of 3.7 mm. FIG. 14F schematicallyillustrates an example cross-section of the fiber 2201 drawn using theassembly 2150 shown in FIG. 14E. In one example of the vacuum fiberdrawing method, the fiber 2201 has an outer diameter of about 125 μm.The fiber 2201 comprises a core 2301 having a diameter of about 1 μm anda first cladding 2302 having a diameter of about 20 μm. The fiber 2201also comprises webs 2303, first additional cladding 2304, secondadditional cladding 2305, and outer cladding 2307.

Certain preferred embodiments of preform, cane, and fiber fabricationmethods have been described above with reference to the figures. It isto be understood that these embodiments are nonlimiting examples ofpossible methods for fabricating an ultra high numerical optical fiber.For example, the phases and techniques described herein may be performeddifferent numbers of times and in different orders in order to fabricatea preform having certain desired properties. Additional processing steps(such as, e.g., the chemical vapor deposition described above) may beused. In some examples of the method, some of the above-described phasesare eliminated and/or combined with other processing phases. Apparatusdifferent from the apparatus 750 shown in FIGS. 7, 10, 14A, and 14D maybe used. Many variations are contemplated.

Embodiments of ultra high numerical aperture fiber can be used in a widerange of systems and applications. Examples of some of these systems andapplications are further described below. It is to be understood thatthese examples are not intended to limit the range of systems andapplications in which UHNAF may be used, and are presented herein forillustrative purposes only.

Many systems, including, for example, telecommunications systems andpulse amplification systems, operate based at least in part on nonlineareffects. One example is a system based on mid-span spectral inversion,which flips the spectrum of optical pulses around its center so thatdispersion in the second part of the transmission can be used tocompress the broadened pulses. Another example is a pulse regenerationscheme based on a nonlinear device, which attenuates preferentially lowamplitude noise. A further example is an ultra-broad-band source basedon super-continuum generation in optical fibers. Each of these systems(as well as others) may benefit from the use of a low-loss UHNAF havinga small modal field diameter (MFD). The small MFD may lead tosubstantially increased nonlinearity. Use of such a UHNAF may alsoreduce a nonlinear device's threshold, insertion loss, and physicalsize.

In another example, an embodiment of a multi-stage telecommunicationsystem 1520 is schematically illustrated in FIG. 15. The system 1520includes a transmitter 1500, one or more fiber spans 1501, one or moreamplifiers 1502, one or more dispersion compensation modules (DCMs)1503, and a detector 1504. Any of the DCMs 1503 may comprise one or morecoils or lengths of UHNAF (such as, e.g., the UHNAF 200 and/or 600 shownin FIGS. 2 and 6A). As described above, the reduced core size and modefield area in the fibers described herein can provide increaseddispersion, for example, dispersion 3 or 4 times the magnitude ofmaterial dispersion. Accordingly, in some embodiments, the DCM 1503comprises a single coil of UHNAF. In certain embodiments, the DCM 1503comprises two (or more) separate coils of fiber, with at least one coilcomprising UHNAF. In certain such embodiments, one coil can be optimizedfor dispersion compensation and another coil can be optimized fordispersion slope compensation for a single span of the transmissionoptical fiber. In one embodiment, the coil comprising UHNAF is used fordispersion slope compensation.

In a fiber chirped pulse amplification (FCPA) system, short opticalpulses are stretched into much longer pulses before being amplified. Theamplified optical pulses are then compressed back to their originalpulse width. This reduces peak intensity in the amplifier and avoidsnonlinear limit in the amplifier. In some FCPA systems, the stretchingratio is limited by a lack of dispersion slope compensation for thethird-order dispersion in the compressor.

Therefore, it may be desirable to provide precise third-order dispersioncontrol in the stretcher to pre-compensate for third-order dispersion ofthe compressor. An FCPA system comprising such a stretcher canpotentially allow two orders of magnitude increase in the stretchingratio, which may translate into two orders of magnitude more pulseenergy from the FCPA system. Details of using photonic crystal fibersfor dispersion management in mode-locked fiber lasers and FCPA systemsare addressed in references such as U.S. Pat. No. 7,113,327 (IM-99;Attorney Docket IM AA.021A), U.S. Patent Publication No. 2005-0041702(IM-105), US Patent Publication No. 2004-0213302 (IM106; Attorney DocketIM AA.023A); U.S. Patent Publication No. 2005-0226278 (IM-108; AttorneyDocket IM AA.025A), U.S. Patent Publication No. 2004-0263950 (IM-100;Attorney Docket IM AA.036A), U.S. Patent Publication No. 2005-0105865(IM-114), and U.S. Patent Publication No. 2005-0111500 (IM-125), each ofwhich is incorporated by reference herein in its entirety.

FIG. 16 schematically illustrates an embodiment of an FCPA system 1620that advantageously may utilize UHNAF. The system 1620 includes a seedlaser 1600, a stretcher 1601, a pre-amplifier 1602, a pulse picker 1603,a power amplifier 1604, and a compressor 1605. The stretcher 1602 maycomprise UHNAF such as, for example, UHNAF 200 or 600 (see, FIGS. 2 and6A). In some embodiments of the stretcher 1602, a single coil of UHNAFis used. In other embodiments, two (or more) separate coils of fiber areused, with at least one coil comprising UHNAF. For example, one coil maybe optimized for dispersion compensation, and the other coil optimizedfor dispersion slope compensation for the compressor 1605 used in theFCPA system 1620. In one embodiment, the coil comprising UHNAF is usedfor dispersion slope compensation.

An advantage of certain embodiments of UHNAF is that the fiber may havevery low loss, which allows a long length of the UHNAF to be used incertain implementations without significant optical losses. Anadditional advantage of certain embodiments is that the UHNAF can bebent into a coil having a small bend radius, which enables a smallpackage to be used for the UHNAF. An embodiment of a compact coil 1720of UHNAF is schematically illustrated in FIG. 17. The coil 1720 includesa compact spool 1700, a coil of UHNAF 1701 having an input end 1702 andan output end 1703. This small coil 1720 can be utilized as a componentof a DCM, such as the DCM 1503 described with reference to FIG. 15.Certain embodiments of UHNAF can be fabricated to have a reasonablysmall modal field diameter and consequently reasonably highnonlinearity. In one embodiment, a highly nonlinear fiber (HNLF)comprises the compact coil 1720 shown in FIG. 17. This embodiment of anHNLF advantageously can be used as a component in devices based onnonlinear effects in order, for example, to lower the device thresholdand to reduce the device physical size.

Embodiments of UHNAF can be configured to provide strong opticalguiding, because of the high NA provided by UHNAF compared toconventional fiber. Accordingly, certain embodiments of UHNAF can bebent around a reasonably sharp corner without suffering significantoptical losses. Certain such UHNAF embodiments are thus beneficial whenused in applications such as, for example, providing high capacitynetwork data connections in homes and offices (e.g., wiring opticalfiber circuits around a home or office in a “fiber to the home” (FTTH)implementation).

Optical fibers are used in many types of sensors. For example, sensorsbased on optical fibers can be used to measure the composition of gasand/or liquid. Some of these sensors utilize the interaction of theevanescent field of an optical mode propagating in the optical fiberwith the gas or liquid to be measured. An optical mode in an opticalfiber with a very small core, for example, comparable to or smaller thanthe wavelength of the propagating light, generally has a substantialpart of its optical energy outside the core, e.g., the optical energyhas a long interaction length. If such a small-core fiber is surroundedby a gas or liquid material, optical energy outside the core caninteract with the material and, when detected, provide information aboutthe material. For example, in some implementations, the long interactionlength of the optical energy in the fiber is used for detecting tracelevels of chemicals in the material which may, for example, be air.

Accordingly, some embodiments of the UHNAF disclosed herein can beconfigured with a small core so that a substantial portion of the guidedoptical energy propagates in the first cladding (e.g., first cladding202 or 602 in FIGS. 2 and 6A). Such UHNAF embodiments can advantageouslybe used in a sensor because of their reasonably long optical interactionlength. FIGS. 18A and 18B schematically illustrate two example sensorsystems 1820 a and 1820 b, respectively. The sensor system 1820 aincludes a broad band optical source 1800, a length of UHNAF 1801, anoptical filter 1802, and an optical detector 1803. In use, a gas orliquid medium is introduced into the first cladding region of the UHNAF1801, where the medium interacts with optical energy propagating overthe length of the optical fiber 1801. An optical absorption spectrum canbe measured by the detector 1803 (e.g., in the passband of the filter1802, which may be tunable in wavelength) and can be used to detect andquantify concentrations of species in the medium. Embodiments of thesensor system 1820 a beneficially can achieve high sensitivity becauseof the long interaction of the UHNAF 1801. The sensor system 1820 bschematically illustrated in FIG. 18B includes a wavelength-tunablelight source 1810, a length of UHNAF 1811, and an optical detector 1812.The sensor system 1820 b can be used to measure an absorption spectrumof a gas or liquid medium introduced into the first cladding region ofthe UHNAF 1811. Embodiments of UHNAF may be used in other types ofsensors as well.

A wide variety of variations of UHNAF and methods of fabrication thereofare possible. Components and features may be added, removed, orrearranged as different configurations are possible. Different materialsmay also be used. Additionally, processing steps may be added, removed,or reordered. For example, one or more refractive index raising dopants(e.g. germanium and/or phosphorus) can be added to the core (e.g., thecore 201 or 601) to further raise the numerical aperture of a UHNAF.Doping silica glass with germanium has the added benefit of increasingnonlinear coefficients of the glass, which can be advantageous forhighly nonlinear fibers (such as those using the compact coil 1720discussed with reference to FIG. 17).

Additionally, a birefringent optical fiber can be fabricated byintroducing ellipticity to the core of the UHNAF. Due to the largenumerical aperture of the UHNAF, a reasonably large amount ofbirefringence can be achieved in a fiber having a core with a relativelysmall amount of ellipticity. Polarizing optical fiber, in which a singlepolarization mode is supported, can also be fabricated due to therelatively large birefringence possible in the ultra high numericalaperture fibers.

Embodiments of the UHNAF described herein may be used in optical cablesfor transmitting optical signals between two or more points. Opticalcables may include one or more optical fibers (or one or more bundles ofoptical fibers) that are surrounded by one or more protective layers.For example, the protective layers may include a polymer buffer and/or ajacket or protective sheath. Optical cables comprising UHNAF can be usedin a wide range of applications including, for example,telecommunications, networking, etc.

The above description of certain preferred embodiments has been given byway of example and is not intended to be limiting. Additionally,although certain advantages have been described, not all such advantagesneed be achieved in each embodiment. For example, one advantage or groupof advantages may be achieved or optimized in a particular UHNAFembodiment, without necessarily achieving or optimizing other possibleadvantages. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various equivalents, changes, and modificationsto the structures and methods disclosed. It is sought, therefore, tocover all such changes and modifications as fall within the spirit andscope of the following claims and equivalents thereof.

1. A method comprising: providing a preform assembly comprising an outertube having a passage extending at least partially therethrough and oneor more hollow inner tubes at least partly within the passage, saidhollow inner tubes having hollow regions therein; and expanding saidhollow inner tubes.
 2. The method of claim 1, wherein expanding saidhollow inner tubes comprises: creating a pressure differential betweensaid hollow inner tubes and said passage in said outer tube; heating aportion of the assembly in order to expand the hollow inner tubes. 3.The method of claim 2, wherein creating a pressure differentialcomprises increasing the pressure in at least one of said hollow regionsof said hollow inner tubes.
 4. The method of claim 2, wherein creating apressure differential comprises decreasing the pressure in said passagein said outer tube.
 5. The method of claim 2, wherein creating apressure differential comprises increasing the pressure in at least oneof said hollow regions of said inner tubes and decreasing the pressurein said passage in said outer tube.
 6. The method of claim 2, whereinsaid heating comprises relatively moving the preform assembly and a heatsource.
 7. The method of claim 1, further comprising coating an innersurface of the outer tube prior to inserting therein said one or morehollow inner tubes.
 8. The method of claim 1, further comprising:inserting the assembly in another outer tube; and further expanding saidhollow regions.
 9. The method of claim 2, further comprising:pressurizing the assembly; and heating a portion of the assembly inorder to expand the assembly.
 10. The method of claim 1, furthercomprising drawing said perform assembly to form an optical fiber. 11.The method of claim 10, wherein said fiber has a diameter of less than200 microns.
 12. The method of claim 8, further comprising drawing saidperform assembly to form an optical fiber.
 13. The method of claim 12,wherein said fiber has a diameter of less than 200 microns.
 14. Themethod of claim 1, further comprising caning said perform assembly toform a rigid rod structure.
 15. The method of claim 14, wherein saidrigid rod structure has a diameter of greater than 1 millimeter.
 16. Anoptical preform fabricated by the method of claim
 1. 17. An opticalfiber fabricated by drawing the optical preform of claim
 16. 18. Anoptical fiber fabricated by the method of claim 1.